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Home , Nanoremediation, Nanotoxicology

International Journal of Environmental Science and Development, Vol. 2, No. 3, June 2011 in Remediation

C.S. Rajan

over the exposure of humans and environment to Abstract- Nanotechnology is a multidisciplinary field that has that may exert deleterious effects. gained significant momentum in recent years. Nanoparticles Toxicological risk assessment demands information on both are the key players that have promised many benefits through exposure and uptake of nanoparticles and their immediate their nano-enabled applications in multiple sectors. This effects once they enter the human system. But, the available review summarizes the use of such as zero valent iron (nZVI) and nanotubes (CNT) in data on these topics are very limited to form conclusions environmental cleanup like ground water remediation for and recommendations [6]. In response to these concerns, drinking and reuse. There are, however, concerns regarding various scientific community is gaining knowledge in the potential risks associated with the use of nanomaterials to exposing their toxicological effects on human [7] and the environment and human health. An understanding of the ecological health [8].This review focuses on various relationship between the properties of nanoparticles and their research and experimental works regarding the use of in vivo effects would provide an effective strategy to tackle the deleterious effects. nanotechnology for environmental cleanup, particularly on their application in ground water and drinking water Index Terms- , Iron Nanoparticles, remediation with the help of nanoparticles such as Zero , . valent iron (nZVI) and Carbon Nanotubes (CNT). It also focuses on the deleterious effects of using these nanoparticles. I. INTRODUCTION

In recent years, nanoscience and technology has introduced a new dimension to scientific disciplines and II. SITE REMEDIATION technology sectors due to its ability to exhibit super- In response to a growing need to address environmental functional properties of materials at nano-dimensions. There contamination, many remediation technologies have been is a remarkable rise in research and development in all developed to treat , leachate, wastewater and developed countries and many developing countries groundwater contaminated by various pollutants, using in pertaining to this field. Organizations such as Universities, situ and ex situ methods [9]. In particular, a contaminated public research institutes and industrial R&D laboratories site may require a combination of procedures to allow the 1focus strongly on this new technology to benefit from its optimum remediation for the prevailing conditions. scientific and technological advantages [1]. Nanotechnology Chemical, physical and biological technologies may be used is a multidisciplinary field that applies engineering and in accordance with one another to reduce the contamination manufacturing principles at molecular level [2]. In broad to a safe level. Hence, for a successful treatment, proper terms, nanotechnology is the development and use of selection, design, and adjustment of the remediation techniques to study physical phenomena and construct technology’s operations should be carried out based on the structures in the physical size range of 1–100 nanometers properties of the contaminants and and on the (nm) as well as the incorporation of these structures into performance of the system [10]. Previously, conventional applications [3]. The past couple of decades have been methods include primarily pump and treat operations. This dedicated to the synthesis, characterization, and application method involves extraction of groundwater through wells of nanomaterials and trenches and treating groundwater by above-ground (ex situ) processes such as air stripping, carbon , Nanotechnology has revolutionized a multitude of sectors biological reactors or chemical precipitation [11]. But such as the electronic, chemical, biotechnology and unfortunately, most of these methods produce highly biomedical industries [4]. Whereas various industries contaminated waste which then has to be disposed off, produce different varieties of nanomaterials there are resulting in high operation time [12]. increasing efforts to use nanotechnology in environmental A common type of in situ or below-ground remediation engineering to protect the environment by pollution control, method used for cleaning up contaminated groundwater is treatment and as a remedial measure to long term problems the permeable reactive barrier (PRB). PRBs are treatment such as contaminated waste sites [5]. This technique has zones composed of materials that degrade or immobilize proved to be an effective alternative to the conventional contaminants as the groundwater passes through the barrier. practices for site remediation. Further research has also They can be installed as permanent, semi- permanent or been carried out and its application is found useful in the replaceable barriers within the flow path of a contaminant treatment of in drinking water. plume. The material chosen for the barrier is based on the Despite their potential benefits, there is a major concern contaminant(s) of concern [11]. One drawback of PRBs is that they can only remediate contaminant plumes that pass Manuscript received April 7, 2011 through them; they do not address dense non aqueous-phase C. S. Rajan is with the Department of Chemical Engineering, liquids NAPLs (DNAPLs) or contaminated groundwater Sathyabama University, Chennai.600119 (e-mail: [email protected]). that is beyond the barrier.

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III. NANOREMEDIATION especially chlorinated organic , organochlorine In recent years, nanoremediation has become the main and polychlorinated biphenyls (PCBs) [18]. focus of research and development. There is great potential The large specific surface area nanoparticles are to use this technology to clean up the contaminated sites and significantly more active than larger particles of the same protect the environment from pollution. This eco-friendly material to remove from ground water [19].This was technology is considered to be an effective alternative to the evident as the nanoparticles were able to bind arsenic five to current practices of site remediation. Nanoremediation ten times more than micro-sized particles. methods involve application of reactive materials for the detoxification and transformation of pollutants. These materials initiate both chemical reduction and catalysis of the pollutants of concern [5]. The unique properties of nanomaterials make them best suited for in situ applications. Their small size and novel surface coatings enable them to achieve farther and wider distribution when compared to large-sized particles [8]. The use of nanotechnology for site remediation could potentially provide a solution for faster and more cost- effective site remediation. Many different nanomaterials Fig.1 Schematic diagram of Zero valent Iron have been evaluated for use in nanoremediation. They include nanoscale zeolites, metal oxides, carbon nanotubes, noble metals and . Of these, nanoscale zero- valent iron (nZVI) is currently widely used in [5]. In addition to groundwater remediation, nanotechnology has also contributed towards reducing the presence of non-aqueous phase liquids (NAPL). For this purpose, a material utilizing nano-sized oxide is used in situ to clean up heating oil spills from underground oil tanks. Compared to previous remediation methods, this approach provided an overall reduction in the contaminant levels [13].

IV. NANOSCALE IRON (NZVI) Iron nanoparticles are an attractive component for nanoremediation. Iron at the nanoscale was synthesized from Fe (II) and Fe (III), using borohydride as the reductant. Nanoscale zero-valent iron particles range from 10 to 100 Fig. 2. Magnified Image of an Emulsified Zero Valent Iron (EZVI) nm in diameter. They exhibit a typical core shell structure. To remove the contaminants, the super paramagnetic The core consists primarily of zero-valent or metallic iron property of iron nanoparticle was manipulated and retrieved whereas the mixed valent [i.e., Fe (II) and Fe (III)] oxide using a magnetic field without the negligence of being shell is formed as a result of oxidation of the metallic iron. released into the environment. Laboratory tests have Iron typically exists in the environment as iron (II) - and indicated that in excess of 99% of arsenic in water samples iron (III)-oxides [14]. nZVI are generally preferred for can be removed using 12nm diameter iron oxide nanoremediation because of large surface area of nanoparticles [20]. nanoparticles and more number of reactive sites than micro- For the treatment of trichloroethane (TCE), a hazardous sized particles [8] and it posses dual properties of organic contaminant present in water, the surface of the zero adsorption and reduction, as shown in figure 1. This enables valent iron nanoparticle is modified to contain an oil-liquid it to be used for the remediation of wide range of membrane. This oil-liquid membrane which is generally contaminants present in situ. Moreover, when zero valent composed of food-grade surfactant, biodegradable oil and iron was allowed greater access to the contaminant site, it water is hydrophobic and forms an with ZVI. This was found to give out less amount of hazardous waste is termed as emulsified zero valent iron (EZVI) [21]. Since during the treatment process [15]. Zero valent iron can also all DNAPLs (dense non-aqueous phase liquids), such as the be modified based on the contaminants present. It could be trichloroethane are hydrophobic, the emulsion is miscible modified to include catalysts like , coatings such with the contaminant, allowing an increased contact as polyelectrolyte or triblock polymers [16] or can be between TCE DNAPL and the ZVI present within the oil encased in emulsified vegetable oil micelles [17]. emulsion droplet [22]. Whereas the ZVI in the emulsion In 2003, nanoscale iron particles were investigated for remains reactive, the chlorinated compounds are their effect on a number of common pollutants in continuously de-chlorinated within the aqueous emulsion groundwater and contaminated soil. The results showed that droplet which produces a concentration gradient within the the nanoscale iron particles were highly effective for the oil membrane, which in turn acts as a driving force to allow transformation and detoxification of a number of pollutants additional TCE migration into the membrane and additional

183 International Journal of Environmental Science and Development, Vol. 2, No. 3, June 2011 degradation is carried out. A potential benefit of EZVI over V. CARBON NANOTUBES (CNT) NZVI for environmental applications is that the hydrophobic membrane surrounding the NZVI protects it In recent years, nanotechnology has introduced from other groundwater constituents, such as some different types of nanomaterials to the water industry and inorganic compounds, that might otherwise react with the has produced some promising outcomes .Since its discovery, NZVI, reducing its capacity or passivating the iron. [23]. carbon nanotubes have attracted great attention due to its Another type of nanoparticle used for environmental unique properties. CNTs are nanomaterials that are rolled application in the Bi metallic nanoparticle (BNP).Bi into a tube and are classified as single-walled carbon metallic nanoparticle consist of elemental iron or other nanotubes (SWNT) and multi-walled carbon nanotubes metals in conjunction with a metal catalyst, such as (MWNTs). platinum (Pt), gold (Au), (Ni), and palladium [22]. According to the carbon atom layers in the walls of the The combination of metals to form a nanoparticle increases nanotubes [30]. Removal of contaminants and recycling of the kinetics of oxidation-reduction () reaction, the purified water would provide significant reductions in therefore catalyzing the reaction. cost, time and labor to the industry and result in improved The most commonly used and commercially available environmental stewardship [31]. One such nanomaterial is BNPs are the Palladium and Iron BNPs (Pd/Fe). The surface the carbon nanotube (CNT). These nanosorbents are area normalized rate constant of BNPs of iron combined increasingly attractive since their discovery, due to their with palladium (NZVI/Pd) was two orders of magnitude exceptional adsorption properties, their ability to be higher than that of MZVI [24]. Palladium and Iron BNPs attached to a functional group to increase the affinity are generally used in the removal of TCE (Trichloroethane). towards the target molecule [31]. In one of the studies, palladium converts TCE into ethane The hexagonal arrays of carbon atoms in graphite sheets with minimal formation of vinyl chloride and other of CNTs surface have a strong interaction with other chlorinated intermediates that often occur with anaerobic molecules or atoms, which make CNTs a promising and with iron metal [25]. adsorbent material substituted for activated carbon in many

ways [32]. They are utilized for the removal of heavy A. Risks metals like Cr3+, Pb2+[33], and Zn2+ [34], metalloids such as

arsenic compounds [35], organics, biological impurities [31], Apart from using nZVI for various classes of and removing many kinds of organic and inorganic contaminants, there is little information or research being pollutants such as dioxin [36] and volatile organic conducted based on the potential toxicological effect they compounds[37]. On comparing CNTs with other adsorbents, might pose. There are insufficient data on the potential for the researchers assert that nCNTs are effective adsorbents accumulation of nanoparticles in environmentally relevant for environmental applications [9]. The carbon nanotubes species [26] and there have been few studies on the effects (CNTs) are unique and one-dimensional macromolecules of many nanoparticles on environmental microbial that posses high chemical and thermal stability [38]. This communities [3]. Under standard environmental conditions property of the nanoparticles have been manipulated for the i.e., using aerated water and pH ranging from 5 to 9, Fe2+ treatment of natural organic matter (NOM) which could will readily and spontaneously oxidizes to form Fe3+ and produce carcinogenic agents [39] and even enhance the precipitate out of the groundwater as insoluble iron oxides bacterial regrowth and bio-film formation [40]. Hence, and oxy-hydroxides. thermally treated CNTs were employed for the treatment of Ongoing studies are evaluating surface coatings and other natural organic matter to realize effective absorption. Thus, modifications that would maximize subsurface mobility of CNTs posses all the essential properties to maintain water nZVI [27]. Whereas increased mobility would allow more of high quality. Here we look at various methods in which efficient remediation, it could also result in the possibility of CNTs can be used effectively to retrieve the various types the nanomaterials migrating beyond the contaminated of contaminants present in water for drinking and for reuse. plume area, seeping into drinking water aquifers or wells or discharging to surface water during the remediation process.

Nanoparticles may have a negative impact on human health when nanoscale particles are inhaled, absorbed through skin or ingested [26]. Because of their small size, the particles have the potential to migrate or accumulate in places that larger particles cannot. One such area is the alveoli of lungs, hence potentially increasing [28]. Problems of toxicity and safety have limited the use of nanotechnology for remediation by some private-sector companies. A work done by DuPont, for example, has ruled out the use of nZVI for site remediation at any of its sites until problems concerning fate and transport have been more thoroughly researched. Their research has cited questions of post remediation persistence and potential Fig. 3. A schematic Representation of Carbon Nanotube and Multi walled human exposure to the particles as areas of particular Carbon Nanotube concern during nanoremediation [29].

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For ground water remediation, there are stringent acceptance for using nanomaterials in water purification regulation methods. Any increase in the discharge of heavy because of their unknown toxicity and environmental metals into the aquatic environment is toxic as they can impact. Scientists have found that carbon nanotubes, if accumulate in the living tissues. Hence they have to be inhaled in large proportions, could be as dangerous as removed from water. This is done using multi-walled [48]. Researchers found carbon nanotubes causing carbon nanotubes (MWCNTs). To increase the absorption the same kind of damage as asbestos in mice. According to capacity of MWCNTs, it is oxidized with nitric acid Lam and its co-workers, CNTs are light and could get air resulting to which a higher level of adsorption was achieved. borne and when it enters the lungs, lesions were formed and According to Li and co-workers (2003), the sorption of Pb toxicity greater than that of quartz was observed [44]. (II), Cu (II) and Cd (II) on to MWCNTs were 3-4 times Another set of experiments reveal that exposure to larger than those of powdered activated carbon and granular nanotubes on mice could cause mesothelioma, a cancer activated carbon which are the two conventionally used which affects the lung lining. All these factors have affected sorbents in water purification. the use of these nanomaterials in nanoremediation by Some studies showed that un-derivatized CNTs tend to various organizations. Hence extreme degree of safety and be water insoluble and toxic. Carbon nanotubes, in order to caution must be maintained when carrying out experiments be highly dispersed in water and to be easily separated from using carbon nanotubes (CNTs). their dispersion for their re-use, are functionalized with various functional groups (e.g. hydroxyl, carboxyl, amines, etc.) to increase their water solubility and biocompatibility VI. CONCLUSION in some cases [41, 42]. In 2006, Jin and co-workers functionalized MWCNTs with Fe nanoparticle for their The aim of this review is to give an overall perspective effective disposal of aromatic compounds which are of the use of nanoparticles to solve potential issues considered as . In addition to this, to make such as treatment of contaminated water for drinking and CNTs water soluble, it is made to decompose thermally reuse more effectively, than through conventional means. with azodiisobutyro nitrile (AIBN) and refluxed with Nanoremediation has the potential to clean-up large sodium hydroxide to form nanoparticle functionalized contaminated sites in-situ, reducing clean up time and water-soluble MWNTs (Fe-MWNTCH2COONa). eliminating the need for removal of contaminants and hence According to the analytical data the adsorbed percentages of reducing the contaminant concentration to near zero. benzene, toluene, dimethylbenzene and styrene were found As highlighted, a great degree of care needs to be taken if to be 79%, 81%, 83% and 88% respectively, which showed it has to be implemented in real life scenarios to avoid that Fe-MWNT-CH2COONa can be used as a potential deleterious effects. The success of the technique in field sorbent for the removal of benzene and its aromatic conditions is a factor of interdisciplinary work that is compounds. Fe-MWNT-CH2COONa could also be reused involved. The collaboration of chemistry, material science due to their exceptional magnetic separation capability [43]. and geology is one of the key challenges of this research.

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